Fossil fuels contain naturally occurring selenium, which exists in several oxidation states, including selenide (−2), elemental selenium (0), selenite (+4), and selenate (+6). Refining of oils and processing of coals containing selenium can generate process water with amounts of selenium-containing compounds in excess of limits allowed by governmental standards for discharge of the water into the environment. These aqueous streams often include the selenium in soluble forms, such as selenocyanate (SeCN−) in zero oxidative states and selenite (SeO3−2) and selenate (SeO4−2) as oxidized species.
Various treatment techniques for the process water provide ways to remove selenium and may rely on adsorption. The techniques often remove all types of the selenium to some extent but with relative less ability to remove the oxidized species of the selenium. Past improvements to selenium removal effectiveness and efficiency focus on materials for sorbents and sorption reaction conditions between the sorbents and the selenium. However, the treatment techniques still may not enable achieving selenium limits at all times and with a sufficient margin from desired or regulated levels. Cleaned water resulting from the treatment techniques further often fails to meet criteria for reuse of the cleaned water in such applications as feed to boilers or desalting units.
Therefore, a need exists for improved methods and systems for removal of selenium from a fluid.